The invention relates to a method for producing a roll for a paper or board machine, and to the roll so produced.
As known from the prior art, some of the rolls in papermaking machines are made out of shell blanks by bending sheets, then after bending of the sheets the outer surface of the shell is turned on a lathe, in which connection the errors in shape caused by manufacturing inaccuracies are passed to variations in wall thickness, wherefore there arise variations in the stiffness of the shell and eccentricity of the centre of gravity of the shell with respect to its centre of rotation. The unbalance caused by the eccentricity of the centre of gravity may be minimized in the blank by proper centering and by balancing of the shell, but it is difficult to correct the stiffness variations later on. Turning causes errors in the roundness and straightness of the roll also because the blank has residual stresses and/or because the thickness of chips during machining varies. The variation of roll stiffness produces so-called semicritical vibration at a speed which is half of the critical speed of the roll. Unbalance in turn causes vibrations at all speeds.
It is an object of the invention to propose a solution to the above-mentioned problems.
In accordance with the invention, the variation of the stiffness of the shell and its need for balancing are substantially reduced because the shell body is not turned on a lathe, but, instead, after a roll blank has been produced out of a sheet by bending, the shell is first covered by a coating and turned only after this. In that connection, the errors in shape are passed to the coating as thickness variation and, as the modulus of elasticity and the density of the coating are lower than those of steel, both the variation of the stiffness and the unbalance of the shell are substantially reduced. As the coating is used, for example, in a dryer section a material that withstands high-temperature conditions. The coating is selected such as to have a low stiffness and a low density, i.e. the density and the modulus of elasticity are low. The invention can be applied to roll blanks made of planar sheets by bending as well as in connection with centrifugally cast rolls.
In accordance with the invention, the thickness of the wall is measured and its shape is determined accordingly such that the difference in stiffness is as low as possible.
The coating can be applied to the surface of the roll by rotational casting or by vulcanizing, after which the roll is machined to a desired shape and to a desired surface quality.
The invention is most suitable for use in connection with guide rolls and nip rolls having a low nip load, such as, for example, a reeling drum.
By means of the invention, improved quality and cost savings are achieved.
In the following, the invention is described in greater detail, and the benefits of the invention are also described with reference to the accompanying drawings, to the details of which the invention is, however, by no means intended to be narrowly confined.